Influenza is one of the infectious diseases most widely spread all over the world, and 250,000 to 500,000 people die of the disease annually. In Japan, 5 to 15% of the population contract influenza annually, and there are cases in which aged individuals or immunocompromised patients who have contracted influenza are complicated with pneumonia and result in death.
Influenza viruses are classified into three groups, namely type A, type B, and type C, based on differences in the antigenicity of protein which constructs a virus particle. Among them, type A and type B are mainly the ones which cause an infection in humans and circulate repeatedly every winter.
Influenza vaccines are used to prevent influenza. Attenuated live vaccines (i.e., in which attenuated viable pathogens are employed), inactivated vaccines (i.e., in which pathogens which lost infectivity after being subjected to inactivation treatment are employed), and component vaccines (i.e., in which purified specific components of pathogens are employed) are used worldwide, among which only component vaccines are practically used in Japan for prevention of influenza viruses.
A strain which is likely to prevail in a current year is predicted based on information of the influenza virus strain which circulated in the previous season, genetic information of the influenza viruses concurrently isolated in other countries, the prevalence of antibody for an influenza virus strain in the population, and the like, and influenza vaccines are produced based on the prediction.
Treatment methods of influenza include pharmacotherapy using an anti-influenza virus agent, and amantadine and a neuraminidase inhibitor (i.e., oseltamivir and zanamivir) are approved as anti-influenza virus agents in Japan (Non-Patent Document 1).
Meanwhile, RNAi (i.e., abbreviation of RNA interference) was found as a means for inhibiting expression of a specific gene in recent years. RNA interference refers to a biological phenomenon of inhibition of expression of a target gene, in which an mRNA, which is a transcription product of a target gene, is specially cleaved by a double-stranded RNA homologous with a specific region of the target gene at a site homologous with the double-stranded RNA (Patent Document 1).
In mammalian cells, introduction of a long-chain double-stranded RNA into a cell induces interferon and causes apoptosis. However, it has been elucidated that an mRNA of a target gene is specifically cleaved without causing apoptosis and thus a function of the target gene can be inhibited by introduction of a short-chain double-stranded RNA having 21 to 23 bp into a cell (Patent Document 2). Here, a short-chain double-stranded RNA which causes RNA interference in mammalian cells is called siRNA (i.e., abbreviation of small interfering RNA).    Patent Document 1: WO1999/32619    Patent Document 2: WO2001/075164    Non-Patent Document 1: Norio SUGAYA, Japanese Journal of Clinical Medicine, 2006, vol. 64, p. 1840-1844